In women, inheritance of a single mutant BRCA1 allele confers a 50-80% lifetime risk of breast cancer [1]. The BRCA1 protein has been implicated in multiple cellular processes, including transcriptional regulation, maintenance of genome stability, chromatin remodeling, protein ubiquitination, cell cycle checkpoint activation, and DNA repair [2]. Given the multifunctional nature of BRCA1, our lab has sought to uncover which of its many activities are responsible for suppressing breast cancer formation. Previously, our lab showed that most, if not all, BRCA1 polypeptides exist in vivo as a stable heterodimer with the BARD1 protein [3]. Formation of the BRCA1/BARD1 heterodimer is vital for the stability and proper localization of BRCA1 [4], as well as its E3 E3 ubiquitin ligase activity [5] [6] and it role in DNA double-strand break repair [7] [8]. Using a mouse model of human breast cancer, our lab found that mammary-specific deletion of either Bard1 or Brca1 results in tumor formation with identical kinetics and histopathology [9], indicating that BARD1 is an essential component of the BRCA1 tumor suppression machinery. Of note, the C-termini of both the BRCA1 and BARD1 proteins each contain two tandem BRCT domains, a phospho-recognition motif found in over 20 human proteins, most of which function in the DNA damage response [11]. The BRCT domains of BRCA1 have been shown to interact, in a mutually exclusive manner, with at least three different phosphoproteins to form distinct BRCA1 complexes. Each of the three phospho-ligands (Abraxas [12], BACH1 [13], and CtIP [14]) has been implicated in homology-directed repair of DNA double-strand breaks, and our lab has demonstrated that the BRCT phospho-recognition activity of BRCA1 is required for tumor suppression [10]. Although the BRCT domains of BARD1 are less well understood, recent studies show that they can bind poly(ADP-ribose) (PAR) [16] as well as phospho-serine peptides [15]. Importantly, germline mutations that disrupt the BARD1 BRCT domains have been identified in many non-BRCA1/BRCA2 breast cancer families [17] [18] [19], indicating that these domains may also be essential for the tumor suppressor activity of the BRCA1/BARD1 heterodimer. Thus, study of the BARD1 BRCT domains and their interacting ligands may uncover new functions of the BRCA1/BARD1 heterodimer and clarify which of its functions are essential for tumor suppression. Accordingly, the specific aims of this proposal will be: (i) to determine whether the phosphoprotein-recognition or PAR-binding ability of the BARD1 BRCT domains is required for suppression of breast cancer, (ii) to determine whether the phosphoprotein-recognition or PAR-binding ability of the BARD1 BRCT domains affects the genome stability functions of the BRCA1/BARD1 heterodimer, and (iii) to identify novel binding partners of the BARD1 BRCT domains and investigate their role in genome stability and tumor suppression.